A conventional CATV broadcasting system is known to provide various types of information over a plurality of channels to its subscribers. The CATV signals from the plurality of channels are processed and transmitted through one of several known transmission modes, e.g., standard (STD), harmonical related carriers (HRC) and interval related carriers (IRC) transmission modes, especially in the North America.
In such a CATV broadcasting system, in order to efficiently modulate and transmit the CATV signals, various carrier or modulation frequencies are assigned to these transmission modes. In the HRC transmission mode, for instance, the modulation frequencies of all channels except the channels of, e.g., 5 and 6, have, e.g., 1.2 MHz frequency offsets lower than those of the STD transmission mode; and the modulation frequencies of channels 5 and 6 are, e.g., 0.75 MHz higher than those of the STD transmission mode. In the IRC transmission mode, on the other hand, the modulation frequencies of all channels except channels of, e.g., 5 and 6, are identical to those of the STD transmission mode; and the modulation frequencies of channels 5 and 6 are, e.g., 2 MHz higher than those for the STD transmission mode.
The CATV signals for the plurality of channels are modulated at a transmitter based on one of the three transmission modes. At the CATV receiver, one of the modulated CATV signals is selected and processed by employing a conventional CATV signal reconstruction technique to provide a reconstructed CATV signal to a next processor, e.g., a display unit, for the display thereof.
The quality of the reconstructed CATV signal may depend on an automatic fine tuning (AFT) technique employed to tune the modulated CATV signal from a selected channel. Consequently, to reduce the amount of time needed to tune the modulated CATV signal and enhance the quality thereof, there have been proposed various AFT techniques.
One of such techniques is disclosed in U.S. Pat. No. 5,329,364, issued to Nam H. Lee, entitled "METHOD FOR REDUCING HRC MODE RECEPTION TIME IN CABLE TELEVISION". In the disclosed method for reducing the tuning time of a CATV signal transmitted in the HRC transmission mode, it is assumed first that the CATV signal from a selected channel has been transmitted in the STD mode; and then, it is checked whether a synchronization (SYNC) signal is detected at a STD mode modulation frequency for that channel. If the SYNC signal is detected, then it is determined that the CATV signal from the selected channel has been transmitted in the STD transmission mode; and, otherwise, it is assumed that the broadcasting has been made in the HRC transmission mode. Finally, a series of fine tuning processes for the selected channel is carried out with respect to the modulation frequency of the determined mode for that channel to thereby provide a fine tuned CATV signal of the selected channel.
However, the prior art CATV signal tuning technique determines the transmission mode for a CATV signal from a selected channel at each time of selecting the channel, resulting in a prolonged processing time.